Project Summary Microbial pathogens utilize virulence factors to subvert or counteract the innate immune response of the host. In turn, the host can exploit the presence of virulence factors to detect pathogens and mount a protective adaptive immune response. Understanding this dual nature of virulence factors remains a goal in the fields of microbial pathogenesis and immunology. Because many pathogens encode multiple virulence factors, an additional important goal is to determine if alteration of innate responses by one virulence factor can shape adaptive immunity to another virulence factor. A multi-factorial virulence mechanism known as type III secretion that is conserved in a large number of Gram-negative bacteria that cause important human diseases is being studied. The type III secretion system (T3SS) functions to deliver effector proteins into host cells. The effector proteins regulate innate and adaptive immune responses to promote bacterial infection and virulence. A T3SS that is critical for virulence in the bacterial pathogen Yersinia, which causes human diseases ranging from plague to gastroenteritis, is the focus of the project. A mouse infection model is used to gain insights into mechanisms used by effectors to subvert innate immune responses. The same infection model is used to understand how the host exploits effectors to generate a protective adaptive immune response. Two observations form the basis of the project. First, the YopM effector is required for production of high systemic levels of the immunosuppressive cytokine IL-10 during the innate immune response in mice infected with Yersinia. Second, mice that survive infection with Yersinia generate a dominant CD8 T cell response to a protective epitope in the effector YopE. The project will address three questions. First, how does YopM induce high systemic levels of IL-10 and does this process contribute to pathogenesis? Second, how do YopE- specific CD8 T cells protect and what features of YopE are important for generating this response? Third, does induction of IL-10 by YopM delay or decrease the generation of a CD8 T cell response to YopE? Successful completion of this project will lead to better understanding of how the interplay between multiple virulence factors in a pathogen can impact the outcome of the host immune response. The knowledge gained from these basic research studies will impact the fields of microbial pathogenesis and immunology, and could be translated into applications such as the development of novel live vaccines that effectively prime adaptive immunity.